Chapter 12. Key Findings

An overview of the findings from the performance comparisons between the five climatic scenarios established for the study can be found in table 36. The “X” symbol indicates differences exist, and they are statistically significant at a 95-percent confidence.

Table 35. Summary of statistical comparisons.

Performance Measure

Deep-Freeze Wet Region

Moderate-Freeze Wet Region

No-Freeze Wet Region

Deep-Freeze Dry Region

Moderate-Freeze Dry Region

Roughness (flexible)

Fatigue/wheelpath cracking (flexible)

X

X

X

X

X

Transverse cracking (flexible)

X

X

X

X

X

Rut dept (flexible)

X

X

X

Roughness (rigid)

X

X

X

Longitudinal cracking (rigid)

X

X

X

X

X

Transverse cracking (rigid)

X

X

Faulting (rigid)

X

X

Following is a list of descriptions of the significant differences noted in table 36:

Differences in flexible pavement roughness between the various climates at 20 years were not found to be significant.

Rutting accumulations in the moderate-freeze regions (both wet and dry) are significantly larger than the no-freeze wet region at 20 years.

The deep-freeze regions (both wet and dry) and no-freeze wet region accumulate significantly larger quantities of fatigue/wheelpath cracking at 20 years compared to the moderate-freeze regions (both wet and dry).

Predictions for flexible pavement transverse cracking at 20 years result in the deep-freeze regions (both wet and dry) having significantly larger quantities than the moderate-freeze regions (both wet and dry). In addition, the moderate-freeze regions (both wet and dry) have significantly larger accumulations than the no-freeze wet region.

Accumulations of rigid pavement longitudinal cracking at 25 years were significantly lower in the no-freeze wet region as compared with all other regions.

At 25 years, rigid pavement transverse cracking was significantly larger for the no-freeze wet region as compared to the deep-freeze wet region. All other comparisons were found to be insignificant.

The magnitude of joint faulting at 20 years was found to be significantly larger in the deep-freeze dry region as compared with the moderate-freeze dry region.

It should be noted that the data in this study do not support the notion that deep frost penetration and multiple FTCs are mutually exclusive. Areas do exist with high freezing indices and large quantities of annual FTCs.

A review of information provided by participating SHAs and relevant literature regarding local adaptations to mitigate frost was performed. There was a large variation in typical cross sections for similar design situations, and no specific treatment was universally used to counter frost effects. Many States with frost penetration did required additional surfacing or the replacement of frost-susceptible soils with frost-free material. In addition, most of the States have adopted Superpave PG binder specifications and mix design procedures. This relatively recent development has to a large extent eliminated local adaptations to materials specifications and mix designs for HMA pavements.

Life cycle cost analysis was conducted using two different approaches. The first approach used initial construction costs that were consistent for each of the five climatic scenarios and were based on a standard roadway design. This resulted in equivalent uniform annual costs that were not significantly different between the regions. The second approach used initial construction costs that varied with the climatic scenario. The typical section for the deep- and moderate-freeze regions included additional frost-free material to represent the mitigation found in the review of SHA information. This cost analysis resulted in the no-freeze region having equivalent uniform annual costs that were lower than the other regions.

The models developed from this study can be used in pavement management system applications as well as to perform local calibration for the NCHRP 1-37A Guide.